Drying apparatus and method



April4, 1967 R. G. GIDLOW 3,311,991

DRYING APPARATUS AND METHOD Original Filed April 2 1960 3 Sheets-Sheet -l l v v IN VEN TOR. AoLF Gu/wwm GmLow F W WtM A TTORNE Y6 April 4, 1957 R. G. GIDLOW 3,311,991

DRYING APPARATUS AND METHOD Original Filed April 1960 5 Sheets-$11881. 2

I N V EN TOR. R01. F Gu/v/vm GIDL 0W ATToR/VE YJ P 4, 1967 R. cs. GIDLOW Q 3,311,991

DRYING APPARATUS AND METHOD Original Filed April 9, 1960 3 Sheets-Sheet 5 IN V EN TOR. IFOZF GUN/VAR Gmww AT'raRA/E 9 United States Patent 3,311,991 DRYING APPARATUS AND METHUD Rolf Gunnar Gidllow, North St. Paul, Minn, assignor to The Pillsbury Company, Minneapolis, Minn, a corporation of Delaware Continuation of application Ser. No. 25,640, Apr. 29, 1960. This application Apr. 20, 1965, Ser. No. 450,267 29 Claims. (Cl. 34-5) This application is a continuation of my earlier copending application Ser. No. 25,640, filed Apr. 29, 1960, and entitled, Method and Apparatus for Vacuum Freeze Drying Liquid Containing Materials by Contact Condensation of Their Released Vapors with a Refrigerated Liquid.

This invention relates to apparatus and method for dehydrating liquid containing material under low temperature nad low pressure conditions. More particularly, this invention relates to apparatus and method for dehydrating liquid containing foodstuffs and the like by freeze drying under vacuum.

The present invention represents an improvement over known freeze drying systems. It combines a vacuum drier with vapor absorbent means in a unitary construction. The present system promotes more efficient drying by minimizing travel distance for the water vapor given off by the material being dried. This is accomplished by cascading a refrigerated absorbent liquid over the walls of the vacuum chamber in which the materials to be dried are placed. This obviates the need for the usual large vapor piping and eliminates the frictional resistance caused by it. The absorbent liquid may be water miscible or water immiscible. It is concentrated by separation of absorbed moisture and continuously recirculated through the system.

The principal object of this invention is to provide a freeze drying apparatus and method characterized by cascading a refrigerated absorbent liquid over the walls of the drying chamber in close proximity to the material being dried.

A further object of this invention is to provide a freeze drying apparatus and method characterized by cascading a refrigerated water miscible absorbent liquid over the walls of a vacuum drying chamber in close proximity to the material being dried.

A still further object of this invention is to provide a freeze drying apparatus and method characterized by cascading a refrigerated water immiscible absorbent liquid over the walls of a vacuum drying chamber in close proximity to the material being dried.

Other objects of the invention will become apparent as the description proceeds.

To the accomplishment of the foregoing and related ends, this invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.

The invention is illustrated by the drawings in which the same numerals refer to corresponding parts and in which:

FIGURE 1 is a perspective and somewhat schematic representation of one form of the freeze drying apparatus according to this invention adapted to the use of a water miscible absorbent liquid, shown with some parts broken away to reveal internal structural details;

FIGURE 2 is a perspective and somewhat schematic representation of another form of the freeze drying apparatus according to this invention adapted to the use of a water immiscible absorbent liquid whose specific gravity is less than those of either water or ice;

FIGURE 3 is a similar perspective view showing schematically the moisture separation portion of still another form of the freeze drying apparatus adapted to the use of a water immiscible absorbent liquid whose specific gravity lies between those of water and ice; and

FIGURE 4 is a similar perspective and schematic representation of the moisture separator portion of yet another form of the freeze drying apparatus adapted to the use of a water immiscible absorbent liquid whose specific gravity is greater than those of either ice or water.

Referring to the drawings, the drying system of this invention in each of its several embodiments includes a housing, indicated generally at 10, enclosing a drying chamber. Housing 10 is provided with a hinged wall or door panel 11 for providing access to the chamber within the housing. The door 11 fits tightly with respect to the housing 10 by means of appropriate gasketing or like sealing means to render the interior drying chamber gastight. The drying chamber is largely occupied by a plurality of spaced parallel horizontal shelves 12 adapted to support trays bearing the material to :be dried. The shelves 12 are positioned with their edges spaced inwardly from the vertical walls of the housing 10.

The shelves 112 are preferably of hollow double-walled construction or provided with integral tubular coils to permit heating by circulation of a heating fluid. An inlet 13 is provided for introduction of hot water or a heated anti-freeze solution for circulation to heat the shelves and trays. The material to be dryed may also be heated by the use of resistance heating elements in the shelves, by radiant heating means disposed below the next shelf above, and the like. The drying housing is provided with an opening 14- through which the drying chamber is connected by means of a suitable conduit 15 to a vacuum source, such as vacuum pump (not shown). Opening 14 is spaced above the floor of the chamber to prevent the condensing liquid from being drawn into the vacuum source.

Each of the inside vertical walls of the chamber housing is provided with a condenser plate or panel 16. These condenser plates may be integral with the housing walls or are spaced inwardly from and parallel to the housing walls. The condenser plates are preferably formed from a heat conductive sheet material and are preferably immune from corrosive attack by the absorbent liquid cascaded over them. If desired, the condenser panels or the walls of housing 10 may be of double-walled construction for circulation of a refrigerating fluid through the panels or walls, or cooling coils may be disposed in the space between the inner housing wall and the condenser plates in contact with the plates.

The condensing surfaces are desirable corrugated to increase the effective surface area. In some instances the condenser panels may be formed from mesh material or perforated sheet material where increased liquid surface area is desired. Louvers or equivalent open shielding means may be disposed between the edges of the shelves and the condensing surfaces in order to avoid any possibility of contamination of the material being dried by contact with the condensing agent. Such shielding means must merely deflect the condensing liquid away from the material being dried without materially affecting the passage of vapor toward the condensing surfaces.

A distributor tube 17 is positioned within housing 10 at the top of the drying chamber. The distributor tube 17 is in the form of a loop having a geometrical shape corresponding generally to the floor plan of the drying chamber, in this instance, generally rectangular. The

distributor tube is position so that one leg of the distributor tube is disposed adjacent the top edge of each of the condenser plates. The distributor tube is provided with a plurality of openings 18 or a continuous narrow slot for distributing absorbent liquid for cascading over the condenser plates.

The distributor tube is supplied with absorbent liquid from a liquid supply tube 19 which passes through the housing wall and connects with the distributor tube. Supply tube ll9 passes through a chilling unit to cool the absorbent liquid to the desired low temperature before introduction to the drying chamber. The chilling unit 20 is supplied through an inlet pipe 21 with a liquified refrigerating gas, such as ammonia or the fluorinated hydrocarbons sold under the brand names Freon and Genetron. As is well known, cooling takes place as a result of evaporation of the liquified gas which is then discharged through outlet 22 to a compressor unit (not shown) for reliquification and reintroduction the chilling unit.

The refrigerated absorbent liquid discharged through the holes 18 of the distributor tube 17 cascades down over the surfaces of the condenser plates 1-5 where it absorbs water vapor given off by the material undergoing dehydration and collects in the bottom of the drying chamber where it is discharged through sump 23 in the floor of the chamber to an outlet conduit 24.

Referring now in particular to FIGURE 1, which is directed to a form of drying system according to the present invention adapted to utilize a water miscible obsorbent liquid as the condensing agent, there is shown means for separating absorbed moisture from the absorbent liquid by evaporation. Outlet tube 24 is connected to a pump 25 which forces the absorbent liquid and absorbed water vapor through a conduit 26 to a heat exchanger 27 Where the cooled liquid passes in heat exchanging relationship with warmer reeoncentrated absorbent liquid being returned to the chilling unit and drying chamber to absorb further moisture. From the heat exchanger 27 the absorbent liquid passes through conduit 28 and into a coil 29 which is contained in an evaporator unit, indicated generally at 31} The evaporator 30 is in the form of a vertical column. The upper portion of the column is surrounded by a steam jacker 31 and the top 32 of the column is open to the atmosphere. The top end of coil 2'9 is open and discharges into the evaporator column. The condensed moisture absorbed by the water miscible condensing agent dilutes it. The diluted absorbent liquid is heated to its normal boiling point in its passage through the heat exchanger and evaporator and the absorbed moisture is discharged as vapor through the open top of the evaporator to the atmosphere. Venting means (not shown) are preferably provided to carry oif the freed vapors. Evaporation of moisture from the diluted mixture has the effect of reconcentrating the absorbent liquid and raising its boiling point. The concentrated absorbent liquid freed from the absorbed moisture is drawn from the bottom of the evaporator column through conduit 33 and passed in heat exchanging relationship through heat exchanger 27 with the chilled solution from the drying chamber. This partially cooled absorbent liquid is then passed through supply tube 19 for passage through the chilling unit 20 and reintroduction into the drying chamber.

The hydrostatic head in the evaporator column and reduced pressure in the drying chamber forces the return of absorbent liquid to the chiller and drying chamber, or, alternatively, additional pump means may be employed. Flow of absorbent liquid into the drying chamber is controlled by motorized valve 34. Valve 34 is operated by a motor 35 actuated in response to a level detector unit 36 (AccuRay-Industrial Nucleonics Corporation) positioned on the outlet conduit 24 below sump 23.

In order to prevent an accumulation of absorbent liquid in the bottom of the drying chamber, further introduction of liquid into the drying chamber is halted when the level in the outlet tube is higher than a predetermined level. At the same time, in order to insure maintenance of reduced pressure in the drying chamber the liquid in the outlet tube is not permitted to fall below a predetermined level. The valve may also be controlled by determining the level of liquid in the bottom of the drying chamber with a float, or with simple contact points using the brine condensing agent as a conducting medium. A drip guard 37 is disposed on the bottom wall of the drying chamber adjacent the door opening to minimize the possibility of liquid drip from the chamber when the door is opened.

Although the housing and drying chamber are shown in the exemplary illustrations as being generally rectangular in shape, the desired objective of minimizing travel distance of water vapor discharged from the material being dried may be equally well or better achieved in a drying chamber of oval or circular plan. The structure of a drying system having a non-rectangular drying chamber would be substantially as described except for the shapes of the housing, trays, condenser plates and distributor tube. A housing of circular plan would require generally circular shelves and trays. Insertion and removal of circular trays holding the materials to be dried would necessitate a door opening approximately equal to the inside diameter of the chamber. Such a door, of necessity, would be arcuate in structure and may be in one or two panels, as desired. At the same time the condenser plates would also be arcuate to correspond to the housing Walls and, of course, the distributor tube in this instance would be circular.

Refering to FIGURES 2, 3 and 4, there are shown alternative freeze drying systems utilizing, as the condensing agent, a liquid which is not miscible with water. Using such a non-Water miscible fluid the condensed water vapor from the material to be dried is absorbed by the condensing agent as ice crystals and the condensing agent and absorbed moisture are withdrawn from the drying chamber in the form of slush. To facilitate separation of the absorbed moisture from the water immiscible condensing fluid, the ice is melted into Water. It will be seen that three phases may exist in the separator means: ice, water and condensing agent.

The particular technique used to separate the condensing fluid from the absorbed moisture depends upon the relative specific gravities of the three phases present in the separator means. It will be seen that there are three possibilities. It is known that water has a greater specific gravity than ice. The specific gravity of the condensing fluid may be less than those of ice and water in which case the fluid will float on ice, which in turn will float on water. If the condensing fluid has a specific gravity intermediate of those of water and ice, then the fluid will be supported by water and in turn will support the condensed ice crystals. Then as the ice crystals are melted into water, the water droplets, being heavier than the condensing llllld, will sink down through the condensing fluid to the water phase. Where the specific gravity of the condensing fluid is greater than those of water and ice, the fluid will support the water phase which in turn supports the ic phase.

Referring now to FIGURE 2, there is shown a freeze drying system adapted to utilize a water immiscible condensing fluid whose density is less than those of either water or ice. The construction of the vacuum drying chamber, distributor means, condensing panels, supporting means for the material to be dried, chilling unit, vacuum. inlet, discharge outlet, etc. are identical in all material respects with the structure described in connection with FIGURE 1. In this form of the drying system, the outlet tube 24, which receives the discharge from sump 23, communicates with the top of a generally vertically disposed.

III

Settling vessel or tank 40. The tank 40 is preferably pro vided with a conical lower portion 4-1 terminating in a cylindrical portion 42 of lesser diameter.

The stream of slush discharged from the drying chamber, in the form of condensing fluid containing absorbed moisture as ice crystals, is introduced into the top of the settling vessel as. Since the condensing liquid is of lesser density than either ice or water, the ice crystals set tle by gravity into the cone l1 and cylinder 42 at the bottom of the settling tank. Heating means, such as a tubular coil 43, wrapped around the cylindrical portion 42 of the settling tank is provided to heat the ice crystals sufliciently to melt them into water. Thus, just suflicient heat is introduced into heating means 43 to raise the temperature of the ice crystals to about 33 F. The resulting water is dis charged through a bottom outlet through a conduit 44 to pump 45. Pump 45 discharges to a pipe or similar conduit 46 for discharge of the condensed moisture to the sewer. Flow of water through pipe 4-6 is controlled by motorized valve 47 which is actuated by a motor at} which in turn is responsive to the level of water in the settling vessel as monitored by a detector 49 (AccuRay-Industrial Nucleonics Corporation).

The liquid condensing fluid, freed from water and condensed ice crystals, is withdrawn from adjacent the top of the settling vessel from an outlet into a conduit 56 to a pump 51 which discharges directly into the condensing agent supply tube 19 for return of the condensing agent through the chiller unit to the drying chamber. To insure against ice crystals being withdrawn accidentally from the top of the settling tank it into the conduit St), a vertical baflle plate 52 is provided in the top of the settling vessel between the top inlet from conduit 24 and the top outlet to conduit b. Battle 52 extends between opposite side walls of the settling vessel immediately adjacent to the top wall thereof. Thus, the condensing fluid which is withdrawn from the settling tank for recirculation to the drying chamber must first flow around the bottom edge of the bafiie plate and must flow upward to the level of the outlet. This virtually insures complete separation of ice crystals from the condensing fluid. The heat applied to the settling vessel is kept to a minimum and the volume of fluid which is heated is restricted as much as possible to avoid raising the temperature of the condensing fluid to any material extent with consequent necessity for recooling.

In FIGURE 3, there is shown a separator system for removing absorbed moisture from a water immiscible condensing agent whose specific gravity lies between those of ice and water. The drying chamber, which is shown in fragmentary form only, is identical in all material respects with that previously described in detail. The separator system of this form of the invention includes a generally vertically disposed settling vessel 66 having a top inlet in communication with the outlet tube 2.4 from the drying chamber. Tube 24 extends below the liquid level in the separating tank to provide a hydrostatic seal between the slightly higher pressure region in the separating tank as compared to the pressure in the drying chamber. Settling vessel 60 is provided with an outlet adjacent the top thereof which communicates with a conduit 61' for withdrawal of ice crystals from the vessel. The settling vessel 60 is also provided with a bottom outlet which communicates with a conduit 62 for withdrawal of the condensing fluid.

All of the absorbed moisture in the condensing fluid as it enters the settling vessel is in the form of condensed ice crystals. The condensing fluid having a greater density than that of ice sinks to the bottom of the settling vessel whereas the ice crystals remain at the top and are withdrawn through conduit 61 to a second settling vessel 63. In order that the ice crystals may be withdrawn from the first settling vessel before they have had an opportunity to melt into water, of necessity, some condensing fluid must also be withdrawn from settling vessel 60 through the 6 top outlet into conduit 61 and the second settling vessel 63.

The second settling vessel 63 is provided with heating means 64 adjacent the inlet at the top thereof which is in communication with conduit 61 to receive the ice crystals and condensing fluid from the first settling vessel. The heating means 64 is disposed adjacent the top wall of the vessel 63. The ice crystals, having the lowest specific gravity of the three phases of material in this vessel, will remain at the top in near proximity to the heating means. lust enough heat is suppiled to melt the ice to Water. This water, being more dense than the condensing fluid, then sinks to the bottom of the settling vessel 63 where it is withdrawn through a bottom outlet where it is discharged into a conduit 65 to pump 66 to the sewer. Flow of liquid through pipe 65 is controlled by means of valve 67 which in turn is actuated by solenoid 68 in response to the water level in the bottom of vessel 63 as monitored by detector 69.

The condensing fluid in settling vessel 63', freed from its condensed ice and having a lesser density than water, floats on top of the water produced by the melting ice and is withdrawn from the top of vessel 63 through an outlet to a conduit '70 to the top of a fluid surge tank 71. A vertical baflie '72 disposed between the side and top walls of vessel 63 insures against accidental discharge of ice crystals into conduit 76 by making it necessary for the condensing fluid to flow below the bottom edge of the baflie and back up to the discharge outlet. In this manner, the condensing agent is freed from any ice crystals or water droplets. The condensing fluid separated from the ice crystals in the first settling vessel 60 flows through conduit 62 to the top of the surge tank '71 where the two streams of condensing agent are reunited. Flow of fluid through conduit 62 is controlled by motorized valve 73 which is actuated by motor M in response to the level of fluid in the vessel 66 as determined by the detector element 75. The surge tank 71 is provided with a bottom outlet in communication with a conduit 76 to pump 77 for recirculating the separated condensing fluid to the fluid supply line 119 for return to the chilling unit and the drying chamber.

In FIGURE 4, there is shown a separator system for removing ice crystals and water from a fluid condensing agent whose density is greater than that of either ice or water. This form of separator includes a generally vertically disposed settling vessel fltl having an inlet intermediate of the top and bottom which is in communication with the outlet conduit 24 from the drying chamber for introduction of the condensing agent and absorbed moisture to the settling vessel. As shown, the vessel 80 is desirably formed from two cones secured together at the peripheries of their bases. Tube 24 extends below the liquid level in vessel 86 to provide a hydrostatic seal. The condensing fluid, being more dense than the ice crystals carried by it, settles to the bottom of the vessel 8b while the ice crystals rise to the top where they are melted into water through the assistance of a heating element 31. The resulting water being less dense than the condensing fluid floates on top of the condensing fluid and is withdrawn through an outlet at the top of the settling vessel 81 into a conduit 82 to pump 83 for discharge to the sewer. The heavier condensing agent is withdrawn through an outlet at the bottom of vessel 80 into conduit 84!- to pump 85 for circulation through the fluid supply line 19 back to the chilling unit and drying chamber. Flow of condensing fluid through the conduit 84 is controlled by motorized valve 86. Valve 66 in turn is actuated by motor 87 which is operated in response to the liquid level in conduit 24 as detected by the detector unit 83.

Three specific structures have been illustrated and described by which entrained condensed moisture may be separated from a water immiscible liquid condensing agent. Other means of separation are available. The

entrained moisture may be separated from the immiscible liquid by evaporation using a system similar to that iilustrated in FIGURE 1 and described in detail for use with a brine absorbent. The systems described in connection with FIGURES 2, 3 and 4 are based upon separation by gravity. The same principle may be used to effect separation by centrifugaticn, wherein the forces of gravity are merely exaggerated. The entrained ice may be melted and the water separated from the water immiscible condensing agent whether the water or the condensing fluid is heavier. Using a refrigerated centrifuge the ice crystals may be separated from the condensing fluid without first melting the ice. Likewise, ice crystals may be filtered from the condensing liquid under refrigerated conditions.

Throughout this application the moisture freed from the material being dried is described as being absorbed by the condensing liquid. Where the condensing agent is a water miscible fluid, such as a brine, this is literally true. Where the condensing agent is a water immiscible fluid a mechanical absorption takes place. The freed moisture condenses as ice crystals on the surface of the water immiscible fluid. Due to turbulence in the moving stream of liquid the ice crystals become distributed through the liquid stream and are carried away from the drying zone entrained in the water immiscible liquid. When the condensing liquid is subjected to one of the several separation treatments the absorbed or entrained moisture taken up by it is released or desorbed.

In the operation of the freeze drying system according to all embodiments of the present invention, the materials to be dehydrated are spread out in thin substantially uniform layers on flat trays corresponding in size and shape to the shelves 12 of the drying chamber. The shelves are closely spaced to receive a plurality of trays while allowing space for transfer of water vapor. Depending upon the particular product, the material to be dried may be frozen prior to insertion in the drying chamber or may be frozen by the action of the vaporizing water under vacuum conditions within the drying chamber. As an example, meat, which is normally kept under refrigerated or frozen conditions, is conveniently frozen prior to introduction to the drying chamber. On the other hand, raw vegetables may conveniently be frozen in the drying apparatus.

When the drying chamber is filled, the door 11 is closed and the chamber sealed. Vacuum is applied through conduit 15 to reduce the pressure to the order of about 0.01 to 3.0 mm. Hg and desirably below about 1.5 mm. A suitable heating medium, such as hot water to which an anti-freeze agent has been added, is introduced through the inlet 13 to circulate through the chamber shelves so that the water remaining in the material to be dehydrated can be rapidly vaporized by sublimation without melting the ice in the frozen material. If the materials to be dried are not frozen at the time of their introduction into the drying chamber, they will be quickly frozen upon evacuation of the drying chamber due to the rapid loss of water by vaporization.

I In order to remove the water vapor produced by vaporization of the ice in the frozen material to be dried, a refrigerated liquid is introduced to the drying chamber and cascaded down over the condensing plates 16. The water vapor, which has an enormous volume under the high vacuum conditions existing in the drying chamber, is immediately condensed by solution in the refrigerated absorbent liquid when a water miscible condensing agent is used, or is condensed as ice crystals when a water immiscible condensing agent is used.

By providing a cascading film of absorbent liquid on each of the chamber walls surrounding the trays of material to be dried, the distance which the expelled water vapor must travel is minimized and the water vapor is absorbed almost as rapidly as it is formed. The condensing liquid containing absorbed and condensed water vapor is drawn off through sump 23 and pumped to the separator means for removal of the absorbed moisture and then recirculated to the drying chamber. In order to maintain the vacuum within the drying chamber, it is necessary that a head of liquid be maintained in the outlet conduit and the flow of absorbent liquid into the chamber is regulated to maintain this condition while at the same time insuring against flooding the drying chamber with absorbent liquid.

In carrying out the method of the present invention utilizing the apparatus illustrated in FIGURE 1 and a water miscible condensing agent, refrigerated absorbent liquid is preferably a suitable brine, such as, for example, lithium bromide or lithium chloride. The brine is of such concentration as to lower the freezing point of the absorbent liquid substantially below the lowest temperature expected to be encountered in the drying chamber. The solution should be such that, even under the conditions of low pressure and temperatures existing in the evacuated drying chamber, there is substantially no vapor liberated from the brine. This concentration will vary dependent upon the particular brine used and the low temperatures expected to be encountered. As an example, lithium chloride may be used in concentrations between about 40 and and lithium bromide may be used in concentrations between about 50 and 62%. As the expelled water vapor from the material being dried is absorbed by the brine solution, the initial brine concentration will decrease as the absorbed moisture dilutes the brine.

The temperature of the brine is raised in the course of its passage through the heat exchanger 27 where it passes in countercurrent heat exchanging flow with the reconcentrated brine being returned from the evaporator for reintroduction to the drying chamber. The temperature of the absorbent liquid is raised further in the evaporator column and the absorbed water vapor from the material being dried in the drying chamber is evaporated and expelled from the open top of the evaporator column either into the atmosphere or to a suitable vent system.

In its passage through the heat exchanger and evaporator column the temperature of the diluted brine is raised to its normal boiling point. Upon reaching the open top of the evaporator the water is evaporated into the atmosphere. This release of water vapor raises the concentration of the brine and raises the boiling point. The top of the evaporator column is maintained at a temperature to release sufficient absorbed moisture to raise the concentration of the brine to the desired level.

The reconcentrated brine is returned to the drying chamber passing first in countercurrent heat exchanging flow with the liquid from the drying chamber in heat exchanger 27. In the course of its travel through the heat exchanger the temperature of the absorbent liquid is reduced to just slightly above the temperature at which it is to be introduced to the drying chamber. The absorbent liquid'is then given a final chill in the course of its passage through the chilling unit 20 before introduction to the drying chamber. The brine is preferably introduced at a temperature between about 10 and 20 F. depending upon the freezing point of the solution at the given concentration.

As an exemplary illustration of the freeze drying process utilizing a water miscible liquid condensing agent, frozen potato flakes spread in a thin layer on suitable trays are introduced to the drying chamber at an initial temperature of about -10 F. The chamber is closed and vacuum is applied to reduce the pressure to less than about 1.2 mm. A lithium bromide brine of concentration is used as the condensing fluid. When the desired reduced pressure condition is reached, brine is introduced into the drying chamber to cascade down over the condenser panels.

The brine is introduced at an initial temperature between about 15 and 19 F. and at a rate of flow of about gallon (315 ml.) per hour for each square inch of exposed condenser plate surface. At the same time the shelves supporting the trays of frozen potatoes are heated from their initial low temperature to about 120 F. during the course of the drying cycle. This heating causes sublimation of ice crystals within the product being dried. With this material and under these conditions, the rate of evaporation is such that the temperature of the condensing fluid rises about 2 in the course of its passage through the drying chamber.

In the course of its downstream passage through the heat exchanger unit, the temperature of the brine, diluted due to the absorbed moisture contained therein, is raised to or just under the boiling point of the brine. This heated material is then passed into the evaporator column the top portion of which is maintained at a temperature of about 289 F. at which temperature the water vapor is released to the atmosphere. The removing of moisture from the brine has the effect of increasing the concentration of the brine and raising its boiling point. This material is then returned to the drying chamber for recirculation.

In the course of its return passage through the heat exchanging unit in countercurrent flow with the diluted brine from the drying chamber, the temperature of the reconcentrated brine is lowered to within about 3 to 5 of the temperature at which it is desired to reinject it into the drying chamber. The brine is passed through the chilling unit to reduce its temperature to the 15 to 19 F. range desired for introduction to the drying chamber. Upon completion of the drying cycle, the circulation of condensing fluid is stopped, the vacuum is relieved to permit the drying chamber to return to normal pressure and the dried product is removed.

The drying methods of the present invention utilizing the apparatus of FIGURES 2, 3 and 4 are carried outwith the use of Water immiscible refrigerated condensing fluids. The use of a liquid water immiscible condenser surface offers a simple practical solution to the otherwise diflicult problem of removing solid ice from condenser surfaces under vacuum. Exemplary water immisible fluids having properties such that they may be utilized as the condensing fluid in the freeze drying system of this invention are the polydimethyl siloxanes. These fluids are available in a wide range of viscosities. They are heat-stable, oxidation-resistant and water repellant. In general, they exhibit low freeze points and very low vapor pressures. They have a relatively flat viscosity-temperature slope. They are highly resistant to permanent breakdown due to shear. They are non-toxic. Polydimethyl siloxanes are available from the Dow-Corning Corporation, Midland, Mich; General Electric Company, and others. Polydimethyl siloxane fluids sold under the designation Dow- Corning 200 are available at varying viscosities and specific gravities such that some are lighter than water or ice and others have densities intermediate of those of ice and r water. Dow-Corning 550 fluid is heavier than either ice or water. Typical properties of these exemplary silicone fluids are as follows:

I0 of either water or ice is further illustrated by the following example:

Dow-Corning 200 olydimethyl siloxane fluid which has a viscosity at 25 C. of 3.0 centistrokes is used as the condensing fluid. This material has a specific gravity at 77 F. of 0.900 and thus is lighter than either ice or Water. The material to be dried is spread in thin layers on the trays which are introduced into the drying chamber and supported by the shelves therein. The chamber is closed and vacuum is applied to reduce the pressure to less than about 1 mm. When the desired reduced pressure conditions are reached, the silicone fluid is introduced into the drying chamber so as to cascade down over the condenser panels. The silicone fluid is introduced at an initial temperature between about 10 F. and 30 F. and at a rate of flow of about gallon per hour for each square inch of exposed condenser plate surface. At the same time heating fluid is introduced into the shelves supporting the trays of product to be dried.

The moisture driven from the product under influence of the heat in the shelves and the low pressure conditions is released as water vapor having large volume. As the vapor contacts the cascading refrigerated silicone fluid, it is immediately condensed as minute ice crystals and absorbed or entrained and carried away with the cascading fluid. The silicone fluid carrying the ice crystals is withdrawn from the drying chamber in the form of a suspension of ice crystals resembling slush and is introduced into a settling tank, as illustrated in FIGURE 2. In the settling tank the heavier ice crystals sink to the bottom. The bottom portion of the settling vessel is heated by a coil of tubing through which warm water is circulated to raise the temperature of the ice to about 33 F., just suflicient to melt the ice into water. This separated moisture is discharged as water from the bottom of the vessel.

The reconcentrated silicone fluid from which the absorbed moisture has been separated is then returned to a chilling unit where its temperature is reduced to the de sired range for reinjection into the drying chamber. The silicone fluid is continuously circulated until the completion of the drying cycle. The vacuum is then relieved to permit the drying chamber to return to normal pressure. The chamber is opened and the product is removed for packaging or other processing.

The drying process utilizing a water immiscible fluid having a specific gravity between those of ice and water is illustrated by the following example:

Dow-Corning 200 polydimethyl siloxane fluid is used as the water immiscible condensing agent. The grade of DC 200 employed has a viscosity of 25 C. of 50 centistrokes and a specific gravity of 77 F. of 0.960 which lies between the specific gravities of ice and Water. The product is placed in the drying chamber, the supporting shelves are heated, the drying chamber is evacuated, and the silicone fluid is chilled to the desired low temperature, all as previously described. The moisture in the form of water vapor released under the low pressure and tempera- I II III Dow-Corning Fluid No 200. 200. 550. Viscosity at 25 C., centistolzes 3.0. 50 100450 Specific Gravity at 77 F 0.900. 0.960 1.07. Four Point; (ASTM 1397-39 Sect. 5-7) 85 F -67 F F. Thermal Conductivity at 77 F. (gum-cal /sec cm. 0., differential/1 cm. thickness). 0.00027 0.00036 0.00035. Boiling Point 158-212 F. at 0.5 482 F. at 0.5

Volatility (Weight Loss after 48 hrs. at temp.) 9% at 482" 1 Surface Tension at 77 F. (dyne/cm.) 19.2 20.8 24.5.

The freeze drying process utilizing a water immiscible liquid condensing agent whose density is less than those ture conditions existing in the drying chamber is absorbed in the refrigerated silicone fluid cascading over the condenser panels in the drying chamber. Because the fluid is immiscible, the moisture condenses as ice. The resulting slush is withdrawn to a separator unit as illustrated in FIGURE 3.

The slush enters a generally vertical tank. The condensed ice crystals carried in a portion of the silicone fluid are withdrawn fro-m the top of the tank to a secondary separator. The remaining ice-free silicone fluid is withdrawn from the bottom of the first tank. The incoming ice crystals and carrier silicone fluid entering the secondary separator are subjected to just suflicient heat to melt the ice. Thus, the ice and its carrier fluid are heated to about 33 F. Upon melting, the water has a density greater than that of the silicone fluid and, accordingly, sinks to the bottom of the secondary separator from which it is withddawn and discharged.

The silicone fluid freed from its absorbed moisture in the secondary separator is withdrawn from the top of that unit and reunited with the silicone fluid from the primary separator and together they are recirculated first to the chiller unit for recooling and then reintroduced to the drying chamber. This procedure is continued until dehydration is substantially complete. At that time the flow of silicone fluid is stopped, the vacuum conditions are relieved, the drying chamber is opened and the dried product is removed.

The freeze drying process utilizing a non-water miscible liquid condensing agent having a specific gravity greater than those of either ice or water is illustrated by the following example:

Dow-Corning 550 fluid is used as the condensing liquid. This material has a specific gravity of 1.07 making it heavier than either ice or water. Conditions in the drying chamber are as previously described. The silicone fluid and condensed moisture in the form of ice crystals is withdrawn as a slush from the drying chamber and introduced to a generally vertically disposed separator between the top and the bottom. The heavier silicone fluid sinks to the bottom of the separator. The lighter ice crystals rise to the top of the separator where they are melted to water to facilitate their discharge. Here, too, heating is held to a minimum and the ice in the top of the separator is heated to about 33 F., sufficient to melt it. The bulk of the silicone fluid is relatively unaffected by the heating means.

The silicone fluid is recirculated to the chilling unit for reintroduction to the drying chamber. After completion of the drying cycle to substantially complete dehydration of the product, the flow of fluid is stopped, the vacuum is relieved, the chamber is opened and the product is removed.

The present invention offers an improved drying system utilizing a vacuum drier with unitary vapor absorbent means and a plurality of moisture separators permitting the use of either water miscible or water immiscible liquid condensing agents. The present system permits simplification of both the construction and operation of the drying apparatus. At the same time high levels of efliciency are maintained.

It is apparent that may modifications and variations of this invention as hereinbefore set forth may be made without departing from the spirit and scope thereof. The specific embodiments described are given by way of example only and the invention is limited only by the terms of the appended claims.

I claim:

1. Apparatus for the freeze drying of moisture containing material which comprises a housing enclosing a chamber, means within the chamber for supporting the material to be dried, vacuum means for reducing the gas pressure within said chamber, condenser panels in said chamber, distributor means adjacent the tops of said condenser panels for distributing a liquid condensing agent over the surfaces of said panels, chilling means for refrigerating said condensing agent prior to introduction to the distributor means, outlet means in the bottom of said chamber for collecting and discharging said condensing agent, separator means in communication with said outlet means for freeing absorbed moisture from said condensing agent, conduit means for returning condensing agent from said separator means through said chilling means to said distributor means, and means for circulating said condensing agent.

2. Apparatus according to claim 1 further characterized in that means are provided for heating the supporting means for the material to be dried within the chamber to assist in expelling moisture from the material to be dried.

3. Apparatus according to claim 1 further characterized in that said condenser panels are provided with channels for the circulation of a cooling fluid to refrigerate the panels.

4. Apparatus according to claim 1 further characterized in that said condenser panels are spaced inwardly from the inside walls of said housing.

5. Apparatus according to claim 1 further characterized in that valve means are provided in said conduit means for returning condensing agent from said separator means to the distributor means within said chamber, said valve means being responsive to the liquid level within the apparatus to control flow of condensing agent to said distributor means at a rate to maintain a liquid seal in the outlet means from the chamber to insure maintenance of desired reduced pressure within said chamber.

6. Apparatus according to claim 1 further characterized in that said separator means includes heat exchanging means for raising the temperature of said discharged condensing agent to above the normal boiling point of moisture absorbed therein by passing it in countercurrent flow against condensing agent freed from absorbed moisture and evaporator means for expelling said absorbed moisture as vapor.

7. Apparatus according to claim 1 further characterized in that said separator means includes a settling vessel, inlet means at the top thereof for introducing condensing agent and absorbed moisture, outlet means at the top of said vessel for withdrawing separated condensing agent therefrom, said outlet means being in communication with said conduit for return of condensing agent to said chamber, baifle means in said vessel between said inlet and outlet, further outlet means at the bottom of said vessel for withdrawing liquified absorbed moisture therefrom and means for heating the lower portion of said vessel to liquify moisture condensed as ice crystals.

8. Apparatus according to claim 1 further characterized in that said separator means includes a first settling vessel, inlet means at the top thereof for introducing condensing agent and absorbed moisture, outlet means at the top of said vessel for withdrawing absorbed moisture condensed as ice crystals along wtih part of said condensing agent, further outlet means at the bottom of said vessel for withdrawing the separated remainder of said condensing agent, a second settling vessel, inlet means at the top thereof in direct fluid communication with the top outlet of said first settling vessel, outlet means at the top of said second settling vessel for withdrawing separated condensing agent therefrom, baffle means in said second settling vessel between its said inlet means and its outlet means, means for heating the top portion of said second settling vessel to liquify moisture condensed as ice crystals, outlet means at the bottom of said second settling vessel for discharging liquified absorbed moisture therefrom, a condensing agent surge tank, inlet means at the top of said surge tank in direct fluid communication with the bottom outlet means from said first settling vessel and the top outlet means from said second settling vessel to receive separated condensing agent therefrom, outlet means at the bottom of said surge tank for withdrawing condensing agent therefrom, said outlet means being in communication with said conduit means for return of condensing agent to said chamber.

p 9. Apparatus according to claim 1 further characterized in that said separator means includes a settling vessel, inlet means to said vessel intermediate of the top and bottom thereof for introducing condensing agent and absorbed moisture thereto, outlet means at the top of said vessel for withdrawing liquified absorbed moisture therefrom, heating means in the top of said vessel to liquify absorbed moisture condensed as ice crystals and outlet means at the bottom of said vessel for withdrawing separated condensing agent therefrom, said outlet means being in communication with said conduit means for return of condensing agent to said chamber.

10. Apparatus for the freeze drying of moisture containing material which comprises a housing enclosing a drying chamber, means Within the chamber for supporting the material to be dried on a plurality of spaced apart horizontal shelves, means for supplying heat to said supporting means for the material to be dried and for vaporizing moisture contained in the material, vacuum means for reducing the gas pressure within said chamber, condenser panels in said chamber adjacent to and spaced inwardly from the inside walls of said housing, said panels being also closely adjacent to the periphery of the supporting means for the material to be dried, distributor means adjacent the tops of said condenser panels for distributing a liquid condensing agent on said panels, said distributing means comprising a perforated tube disposed adjacent the inside top wall of said drying chamber with segments adjacent to the top edges of said condenser panels, chilling means for refrigerating said condensing agent prior to introduction to said distributor means, said chilling means being provided with an inlet for the introduction of a liquified refrigerating gas and an outlet for discharge of said refrigerating gas, outlet means in the bottom of said drying chamber for collecting and discharging said condensing agent and absorbed moisture, separator means in communication with said outlet for freeing absorbed moisture from said condensing agent, conduit means for returning condensing agent from said separator means through said chilling means to said distributor means, and pump means for circulating said condensing agent through the apparatus.

11. Apparatus according to claim further charac terized in that said separator means includes heat exchanging means for raising the temperature of said discharged condensing agent diluted by absorbed moisture to a temperature approaching the normal boiling point of the diluted condensing agent by passing it in countercurrent flow against condensing agent freed from absorbed moisture and a vertical evaporator column in communication with said heat exchanging means for receiving heated discharged diluted condensing agent therefrom, said evaporator column being open to the atmosphere for expelling heated absorbed moisture as vapor, and conduit means extending from said evaporator column through said heat exchanging means and communicating with said conduit means for returning condensing agent from the separator means to the drying chamber.

12. Apparatus according to claim 10 further characterized in that said separator means includes a generally vertically disposed settling vessel, inlet means at the top thereof for introducing condensing agent and absorbed moisture, outlet means at the top of said vessel for withdrawing separated condensing agent therefrom, said outlet means being in communication with said conduit for return of condensing agent to said drying chamber, bafile means in said vessel between said inlet means and outlet means, further outlet means at the bottom of said vessel for withdrawing liquified absorbed moisture therefrom and means for heating the lower portion of said vessel to liquify moisture condensed as ice crystals.

13. Apparatus according to claim 10 further characterized in that said separator means includes a first generally vertically disposed settling vessel, inlet means at the top thereof in communication with the outlet means from said drying chamber for introducing condensing agent and absorbed moisture, outlet means at the top of said vessel for withdnawing absorbed moisture condensed as ice crystals along with part of said condensing agent, further outlet means at the bottom of said vessel for Withdrawing the separated remainder of said condensing agent; a second generally vertically disposed settling vessel, inlet means at the top thereof in direct fluid communication with the top outlet means of said first settling vessel, outlet means at the top of said second settling vessel for withdrawing separated condensing agent therefrom, baf he means in said second settling vessel between said inlet means and outlet means, means for heating the top portion of said second settling vessel to liquify moisture condensed as ice crystals, outlet means at the bottom of said second settling vessel for discharging liquified absorbed moisture therefrom; a generally vertically disposed condensing agent surge tank, inlet means at the top of said surge tank in direct fluid communication with the bottom outlet means from said first settling tank and the top outlet means from said second settling vessel to receive separated condensing agent therefrom, outlet means at the bottom of said surge tank for withdrawing condensing agent therefrom, said outlet means being in communication with said conduit means for return of condensing agent to the drying chamber.

14. Apparatus according to claim 10 further characterized in that said separator means includes a generally vertically disposed settling vessel, inlet means to said vessel intermediate of the top and bottom thereof in communication with the outlet means from said drying chamber for introducing condensing agent and absorbed moisture to the settling vessel, outlet means at the top of said vessel for Withdrawing liquified absorbed moisture therefrom, heating means in the top of said vessel to liquify absorbed moisture condensed as ice crystals and outlet means at the bottom of said vessel for withdrawing separated condensing agent therefrom, said outlet means being in communication with said conduit means for return of condensing agent to the drying chamber.

15. The method of freeze drying moisture containing material which comprises subjecting the material to a zone of reduced pressure, lowering the tempenature of the material by evaporation of moisture therein under effects of said reduced pressure, distributing a refrigerated liquid condensing agent around the periphery of said reduced pressure zone and cascading said condensing agent over surfaces in close proximity to said material to be dried, absorbing moisture vapor freed from said material in said cascading condensing agent, withdrawing said condensing agent and absorbed moisture from said zone of reduced pressure, separating the absorbed moisture from said condensing agent, recirculating said condensing agent for redistribution to said reduced pressure zone and recooling said recirculated condensing agent just prior to introduction to said reduced pressure zone.

16. The method according to claim 15 further characterized in that the material to be dried is heated While in the zone of reduced pressure to assist in expelling moisture from the material while simultaneously subjected to the chilling effect of the cascading refrigerated condensing agent.

17. The method according to claim 15 further characterized in that the material to be dried is frozen prior to introduction into said reduced pressure zone.

18. The method according to claim 15 further characterized in that said reduced pressure zone is maintained at a pressure of the order of about 0.01 to 3.0 mm. Hg.

19. The method according to claim 15 further characterized in that said condensing agent is a water miscible liquid and said moisture vapor is absorbed into said condensing agent by solution.

20. The method according to claim 19 further characterized in that said absorbed moisture is separated from life said condensing agent by heating the condensing agent diluted by the absorbed moisture to above the normal boiling point of the diluted condensing agent and evaporating by releasing to atmospheric pressure.

21. The method according to claim 20 further characterized in that the condensing agent diluted by the absorbed moisture is heated to a temperature approaching the normal boiling point of the diluted condensing agent by passing in heat exchanging countercurrent flow with recirculating condensing agent.

22. The method according to claim 19 further characterized in that the water miscible liquid condensing agent is a brine formed as an aqueous solution of a salt selected from the class consisting of lithium bromide and lithium chloride.

23. The method according to claim 15 further characterized in that the condensing agent is a water immiscible liquid and said moisture vapor is absorbed into said condensing agent as ice crystals.

24. The method according to claim 23 further characterized in that the water immiscible liquid condensing agent has a specific gravity less than those of ice and water and said absorbed moisture is separated by gravity and withdrawn from beneath said condensing agent.

25. The method according to claim 23 further characterized in that the water immiscible liquid condensing agent has a specific gravity between those of ice and water and said absorbed moisture is removed by melting the ice crystals to water, separating by gravity and withdrawing from beneath said condensing agent.

26. The method according to claim 23 further characterized in that the water immiscible liquid condensing agent has a specific gravity greater than those of ice and Water and said absorbed moisture is separated by gravity and the condensing agent is Withdrawn from beneath the water and ice.

27. The method according to claim 23 further characterized in that the water immiscible liquid condensing agent is a polydimethyl siloxane.

28. The method of drying moisture containing material which comprises subjecting the material to a zone of reduced pressure, lowering the temperature of the material by evaporation of moisture therein under effect of said reduced pressure, introducing a refrigerated liquid water immiscible condensing agent of low vapor pressure into the Zone of reduced pressure into close surrounding proximity with said material to be dried to minimize the travel distance of water vapor given off by the material, absorbing moisture freed from said material in said water immiscible condensing agent, withdrawing said water immiscible condensing agent and absorbed moisture from said zone of reduced pressure and separating the absorbed moisture from said condensing agent.

29. The method according to claim 28 further characterized in that said water immiscible condensing agent is a liquid poly-dimethyl siloxane.

References Cited by the Examiner UNITED STATES PATENTS 2,249,624 7/1941 Bichowsky 34-27 2,310,399 2/1943 Cox 202 52 2,507,632 5/1950 Hickman 345 2,631,018 3/1953 Kals 261-3 2,856,166 10/1958 Goettl 261-29 2,856,699 10/1958 Frey 34-75 2,954,822 10/ 1960 Keville 15926 3,132,929 5/1964 Thuse 34-92 WILLIAM J. WYE, Primary Examiner. 

15. THE METHOD OF FREEZE DRYING MOISTURE CONTAINING MATERIAL WHICH COMPRISES SUBJECTING THE MATERIAL TO A ZONE OF REDUCED PRESSURE, LOWERING THE TEMPERATURE OF THE MATERIAL BY EVAPORATION OF MOISTURE THEREIN UNDER EFFECTS OF SAID REDUCED PRESSURE, DISTRIBUTING A REFRIGERATED LIQUID CONDENSING AGENT AROUND THE PERIPHERY OF SAID REDUCED PRESSURE ZONE AND CASCADING SAID CONDENSING AGENT OVER SURFACES IN CLOSE PROXIMITY TO SAID MATERIAL TO BE DRIED, ABSORBING MOISTURE VAPOR FREED FROM SAID MATERIAL IN SAID CASCADING CONDENSING AGENT, WITHDRAWING SAID CONDENSING AGENT AND ABSORBED MOISTURE FROM SAID ZONE OF REDUCED PRESSURE, SEPARATING THE ABSORBED MOISTURE FROM SAID CONDENSING AGENT, RECIRCULATING SAID CONDENSING AGENT FOR REDISTRIBUTION TO SAID REDUCED PRESSURE ZONE AND RECOOLING SAID RECIRCULATED CONDENSING AGENT JUST PRIOR TO INTRODUCTION TO SAID REDUCED PRESSURE ZONE. 